Mastocytosis is a disease of disordered mast cell proliferation. It affects all ages, both sexes, and all ethnic groups. In some cases, mastocytosis has an aggressive and ultimately fatal course. Thus, our efforts are directed to improving diagnosis and treatment; and to clarifying the etiology of this disease. A successful approach to treatment for aggressive disease remains elusive. However, women with indolent mastocytosis have normal pregnancies and deliveries and their children are not affected. Studies on c-kit and the relevance of activating mutations which we first identified (Asp816Val; Asp816Tyr) in mastocytosis patients are continuing. Data now indicates all adult patients with mastocytosis appear to have the point mutation at position 816 in some peripheral blood cells and in skin lesions. In contrast, pediatric patients with mastocytosis may demonstrate this mutation only in skin lesions; and in some children no mutation in c-kit can be identified. Molecular studies are consistent with the conclusion that the Asp816Val mutation is a somatic mutation and is not in germ line tissues. Plasma levels of CD117 (KIT) and CD25 are useful surrogate markers of disease activity; and aid in the selection of children who should undergo a bone marrow biopsy.